As excitement about the Aug. 21 solar eclipse heats up, experts are urging Montrealers to take good care of their eyes when they enjoy the spectacle in the skies.

The eclipse will fully obscure skies in parts of the United States while partially blocking the sun in Canada. Such partial overlaps of the moon and sun are all the more dangerous because the sun remains exposed to people viewing the eclipse.

“Because it’s not completely blocked, we’re never safe from the damaging rays of the sun,” says ophthalmologist Dr. Cynthia Qian.

Damage from looking directly at the sun can be incurred in just a few seconds.

“The problem with the back of the eye is that there’s no pain sensors, so all this damage can occur without you even knowing about it until it’s far too late,” says Ralph Chou, a University of Waterloo optometry professor and president of the Royal Astronomical Society of Canada.

Sunglasses allow thousands of times more sunlight than is safe to reach your eyes. NASA says special solar eclipse glasses should be marked with the “ISO 12312-2” international safety standard on the label.

Qian adds that looking directly at the sun through the aperture of a phone, camera or telescope is also unsafe.

“Those systems are like when you use a magnifying glass and you can burn a page of paper.”

Specialized adapted filters are available for some optical devices. Wearing eclipse glasses while looking through a camera or telescope aperture is not a suitable alternative; those devices can burn or melt the sunglasses, thereby affecting their effectiveness.

As the eclipse, special glasses that actually offer the requisite protections are becoming increasingly difficult to obtain.

This week, Amazon pulled potentially shady glasses from its site and issued refunds to customers who had already purchased them. In an email to buyers, the company said it could not get confirmation from the supplier that the glasses came from a recommended manufacturer.

“We recommend that you DO NOT use this product to view the sun or the eclipse,” the email said.

With ads for eclipse glasses for sale on Kijiji and social media, it can be hard for armchair astronomers to track down where they were made and whether they meet the standards recommended by NASA.

The Canadian Press asked the University of Toronto to test two pairs of eclipse glasses using an ellipsometer. The machine sends a wave of UV and visible light though the glasses’ lens and uses a sensor to measure how much of the light makes it through.

The first pair, bought at a store recommended by the American Astronomical Society, blocked out more than 99.9 per cent of UV and visible light. The pair was labelled as conforming to the ISO 12312-2 standards for direct observation of the sun.

The second pair, purchased online, did not have the ISO mark and did not perform as well. It let through more than 0.1 per cent of visible and UV light.

The ISO standard allows a maximum of 0.0032 per cent of light to be allowed to pass though. The equipment used could not give a sensitive enough reading to verify if the glasses conformed to the standard.

In both cases, the machine was not able to determine if the glasses met the ISO 12312-2 standard. Both pairs did let through less than one one-thousandth of the light that you’d see with the naked eye.

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“Results show us that the two sets of glasses are most likely safe for observing the sun,” said Herman Wong, a University of Toronto engineering and photonics PhD student who tested the glasses.

He said the machine found that there was almost zero transmission of light that passed through the lenses, but added that there’s really no way to know if they meet NASA recommendations, especially if they are not labelled.

“They are not standardized by any means, so use at your own risk,” he said.

Chou, who has travelled as far as Africa and Asia to view more than 25 eclipses in person, says you shouldn’t let fear deter you from enjoying the event as long as you take precautions. He said an eclipse looks different every time depending on the sun’s flames that day.

“When you see the sun disappear and the corona pop out into view, it is a really awe-inspiring sight. Even for a lot of us scientists … we all get that sort of emotional rush of seeing the corona because it’s really awe-inspiring,” he said.

“This is sort of the giant astronomical clock at work and it’s just the way the universe ticks, or at least the solar system ticks as you see the movement of the Earth around the sun.”

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